Medical Definition of Endothelium

Endothelium: A layer of flat cells lining the closed
internal spaces of the body such as the inside of blood vessels and lymphatic vessels (that convey the lymph, a milky fluid) and the heart.

By contrast, the outside layer of cells that
covers all the free, open surfaces of the body including the skin, and mucous membranes that communicate with the outside of the body is called the epithelium.

The word "endothelium" is derived from the Greek "endon," within + G. "thele," nipple.